Polycrystalline superhard construction

ABSTRACT

A polycrystalline superhard construction comprises a body of polycrystalline superhard material, and a substrate of hard material bonded thereto along an interface. The body of polycrystalline superhard material comprises a first region abutting the substrate along the interface and a second region bonded to the first region. The second region defines a rake face, a cutting edge, a chamfer and at least a part of a flank face, the cutting edge being defined by an edge of the flank face joined to the chamfer, the chamfer extending between the cutting edge and the rake face. The height of the chamfer in a plane parallel to the plane through which the longitudinal axis of the polycrystalline superhard construction extends is less than the thickness of the second region. The first region comprises a material having coarser grains than the second region. There is also disclosed a method of making the same.

FIELD

This disclosure relates to a cutter comprising a superhard construction,particularly but not exclusively for a rotary drill bit for boring intothe earth.

BACKGROUND

Polycrystalline diamond (PCD) material comprises a mass of inter-growndiamond grains and interstices between the diamond grains. PCD materialmay be made by subjecting an aggregated mass of diamond grains to a highpressure and temperature in the presence of a sintering aid such ascobalt, which may promote the inter-growth of diamond grains. Thesintering aid may also be referred to as a catalyst material fordiamond. PCD material may be formed on a cobalt-cemented tungstencarbide substrate, which may provide a source of cobalt catalystmaterial for sintering the PCD material.

PCD material may be used in a wide variety of tools for cutting,machining, drilling or degrading hard or abrasive materials such asrock, metal, ceramics, composites and wood-containing materials. Forexample, tool inserts comprising PCD material are widely used in drillbits used for boring into the earth in the oil and gas drillingindustry. In many of these applications, the temperature of the PCDmaterial may become elevated as it engages rock or other workpiece orbody with high energy. The working life of tool inserts may be limitedby fracture of the superhard material, including by spalling andchipping.

In use as a cutting element in tools such as those mentioned above, thebody of PCD material normally wears according to the followingprogression: smooth wear, woody wear, accelerated wear, spalling.Spalling usually occurs when the wear scar reaches the top workingsurface, and results in catastrophic wear failure.

As used herein, the term “barrel chipping” refers to chipping in thebody of PCD material below a main wear-scar.

Smooth wear as used herein refers to wear occurring at the diamond grainlevel where individual grains or fractions of grains are removed.

Woody wear as used herein refers to the regime where the wear-scarbecomes irregular at the edges and cracking visible. The roughappearance of the wear-scar is possibly due to wear processes at a scaleof more than one grain.

As used herein the term spalling refers to catastrophic failure due towear cracks propagating to top of the PCD body acting as a cutter table.

Durability here refers to distance cut before cutter failure.High-durability cutters tend to maintain cutting integrity buteventually become ineffective due to formation of a very large wear-scarand hence impractical load application requirements. Prevention ofspalling would increase lifetime/durability of the cutter and there istherefore a need for a product in which spalling is partially orcompletely inhibited and a method of producing such a cutter.

SUMMARY

Viewed from a first aspect there is provided a polycrystalline superhardconstruction comprising:

-   -   a body of polycrystalline superhard material;    -   a substrate of hard material bonded to the body of        polycrystalline superhard material along an interface;    -   wherein the body of polycrystalline superhard material comprises        a first region and a second region, the first region abutting        the substrate along the interface and the second region being        bonded to the first region along a further interface,    -   the second region defining a rake face, a chamfer, a cutting        edge, and at least a part of a flank face, the cutting edge        being defined by an edge of the flank face joined to the        chamfer, the chamfer extending between the cutting edge and the        rake face;    -   the first region having a first thickness and the second region        having a second thickness;    -   the chamfer having a height in a plane parallel to the plane        through which the longitudinal axis of the polycrystalline        superhard construction extends, the height of the chamfer being        less than the thickness of the second region;    -   the first region comprising a material having coarser grains        than the material of the second region.

Viewed from a second aspect there is provided a cutter for boring intothe earth comprising the above-mentioned polycrystalline superhardconstruction.

Viewed from a third aspect there is provided a PCD element for a rotaryshear bit for boring into the earth, for a percussion drill bit or for apick for mining or asphalt degradation, comprising the above-describedpolycrystalline superhard construction.

Viewed from a fourth aspect there is provided a drill bit or a componentof a drill bit for boring into the earth, comprising the above-describedpolycrystalline superhard construction.

Viewed from a fifth aspect there is provided a method for making apolycrystalline superhard construction, the method comprising:

-   -   providing a first plurality of aggregate masses comprising        diamond grains having a first mean size, at least one second        aggregate mass comprising diamond grains having a second mean        size; arranging the first aggregate mass on the second aggregate        mass to form a pre-sinter assembly together with a body of        material for forming a substrate; the first region comprising a        material having coarser grains than the material of the second        region; and    -   treating the pre-sinter assembly in the presence of a catalyst        material for diamond at an ultra-high pressure and high        temperature at which diamond is more thermodynamically stable        than graphite to sinter together the diamond grains and a        substrate bonded thereto along an interface to form an integral        PCD construction comprising a first region of PCD bonded to a        second region of PCD, the first region being bonded to the        substrate; the first region having a first thickness and the        second region having a second thickness;    -   the second region defining a rake face, a cutting edge, and at        least a part of a flank face;    -   the method further comprising:

forming a chamfer in the flank face, the cutting edge being defined byan edge of the flank face joined to the chamfer, the chamfer extendingbetween the cutting edge and the rake face, the chamfer having a heightin a plane parallel to the plane through which the longitudinal axis ofthe superhard construction extends, the height of the chamfer being lessthan the thickness of the second region.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments will now be described by way of example andwith reference to the accompanying drawings, in which:

FIG. 1 is schematic partial cross-section through a first embodiment ofa cutter;

FIG. 2 is a schematic partial cross-section through the cutter of FIG. 1showing progression of a wear scar;

FIG. 3 is a schematic partial cross-section through the cutter of FIG. 1showing further progression of a wear scar;

FIG. 4 a is a side view of a conventional cutter showing the wear scarfor a predetermined number of passes;

FIG. 4 b is a side view of an embodiment of a cutter showing the wearscar for the same predetermined number of passes as that applied to thecutter of FIG. 4 a;

FIG. 5 a is a side view of the conventional cutter of FIG. 4 a after afurther number of passes at which spalling has occurred;

FIG. 5 b is a side view of the embodiment of a cutter shown in FIG. 4 bafter the same number of passes applied to the conventional cutter ofFIG. 5 a;

FIG. 6 is a side view of the embodiment of the cutter of FIG. 4 b aftera further number of passes;

FIG. 7 a is a side view of a further conventional cutter after apredetermined number of passes;

FIG. 7 b is a side view of a further embodiment of a cutter showing thewear scar for the same predetermined number of passes as that applied tothe cutter of FIG. 7 a;

FIG. 8 a is a side view of the conventional cutter of FIG. 7 a after afurther number of passes at which spalling has occurred;

FIG. 8 b is a side view of the embodiment of a cutter shown in FIG. 7 bafter the same number of passes applied to the conventional cutter ofFIG. 7 a; and

FIG. 9 is a side view of the embodiment of the cutter of FIG. 7 b aftera further number of passes.

DETAILED DESCRIPTION OF EMBODIMENTS

As used herein, a “superhard material” is a material having a Vickershardness of at least about 25 GPa. Diamond and cubic boron nitride (cBN)material are examples of superhard materials.

As used herein, a “superhard construction” means a constructioncomprising polycrystalline superhard material or superhard compositematerial, or comprising polycrystalline superhard material and superhardcomposite material.

As used herein “polycrystalline superhard” (PCS) material comprises amass of grains of a superhard material and interstices between thesuperhard grains, the content of the superhard grains being at leastabout 50 percent of the material by volume. The grains may comprisediamond or cubic boron nitride (cBN).

As used herein, polycrystalline diamond (PCD) is a PCS materialcomprising a mass of diamond grains, a substantial portion of which aredirectly inter-bonded with each other and in which the content ofdiamond is at least about 80 volume percent of the material with“interstices” or “interstitial regions” between the diamond grains ofPCD material.

As used herein, polycrystalline cubic boron nitride (PCBN) material is aPCS material comprising a mass of cBN grains dispersed within a wearresistant matrix, which may comprise ceramic or metal material, or both,and in which the content of cBN is at least about 50 volume percent ofthe material. In some embodiments of PCBN material, the content of cBNgrains is at least about 60 volume percent, at least about 70 volumepercent or at least about 80 volume percent. Embodiments of superhardmaterial may comprise grains of superhard materials dispersed within ahard matrix, wherein the hard matrix preferably comprises ceramicmaterial as a major component, the ceramic material preferably beingselected from silicon carbide, titanium nitride and titaniumcarbo-nitride.

A cutter 1 according to a first embodiment is shown in FIGS. 1 to 3. Thecutter 1 comprises a substrate 2 bonded along an interface 3 to a bodyof polycrystalline diamond (PCD) material 4. The body of PCD materialcomprises a first region 6 of PCD material bonded to the substrate 2 anda second region 8 of PCD material bonded to the first region 6 along afurther interface 10. The exposed surface 12 of the second region 8forms a rake face 14, a chamfer 20 extending between the rake face 14and a cutting edge 16, and at least a part of a flank 18 of the cutter1, the cutting edge 16 being defined by the edge of the chamfer 20 andthe flank 16.

The “rake face” 14 of the cutter 1 is the surface or surfaces over whichthe chips of material being cut flow when the cutter 1 is used to cutmaterial from a body, the rake face 14 directing the flow of newlyformed chips, and is commonly referred to as the top face of the cutter.As used herein, “chips” are the pieces of a body removed from the worksurface of the body by the cutter 1 in use.

As used herein, the “flank” 18 of the cutter 1 is the surface orsurfaces of the cutter 1 that passes over the surface produced on thebody of material being cut by the cutter 1 and is commonly referred toas the side or barrel of the cutter. The flank 18 may provide aclearance from the body and may comprise more than one flank face.

As used herein, a “cutting edge” 16 is intended to perform cutting of abody in use. A “rounded cutting edge” is a cutting edge that is formedby a rounded transition between the rake face and the flank.

As used herein, a “wear scar” is a surface of a cutter formed in use bythe removal of a volume of cutter material due to wear of the cutter. Aflank face may comprise a wear scar. As a cutter wears in use, materialmay be progressively removed from proximate the cutting edge, therebycontinually redefining the position and shape of the cutting edge, rakeface and flank as the wear scar forms. As used herein, it is understoodthat the term “cutting edge” refers to the actual cutting edge, definedfunctionally as above, at any particular stage or at more than one stageof the cutter wear progression up to failure of the cutter, includingbut not limited to the cutter in a substantially unworn or unused state.

With reference to FIGS. 1 to 3, the chamfer 20 is formed in thestructure adjacent the cutting edge 16 and flank 18. The rake face 14 istherefore joined to the flank 18 by the chamfer 20 which extends fromthe cutting edge 16 to the rake face 14, and lies in a plane at apredetermined angle θ to the plane perpendicular to the plane in whichthe longitudinal axis of the cutter 1 extends. In some embodiments, thischamfer angle is up to around 45 degrees.

The interface 10 between the first and second regions of PCD material, 6and 8, is spaced from the cutting edge 16 which is defined by the secondregion 8. Therefore, the thickness of the second region 8 is greaterthan the vertical height of the chamfer 20.

The thickness of the first region 6 of PCD material is substantiallygreater than the thickness of the second region 8. For example, in someembodiments, the thickness of the second region 8 is up to around 600microns and the thickness of the first region 6 is around 1200-1800microns. In some embodiments, the thickness of the second region 8exceeds the vertical chamfer height by around 100-400 microns and thevertical height of the chamfer 20 may be, for example, around 400microns.

In FIGS. 1 to 3, the hashed lines 22, 24 represent the work face makingan angle φ with the longitudinal axis of the cutter 1. This angle φ isalso referred to as the back-rake angle.

As the cutter 1 wears, the wear on the cutter 1 is shown by a shift inthe hashed line 22 to the position denoted by the second hashed line 24.FIG. 1 shows the first stage where all cutting is carried out by thesecond region 8 of the body of PCD material. The first hashed line 22shows the start of the cut and the second hashed line 24 shows where thewear has reached the interface 10 between the first and second regions6, 8 of PCD material. The initial back rake angle φ and its progressshift is thereby shown in FIG. 1 as the cutter 1 wears during use, thewear-flat eventually reaching the thicker/softer PCD layer of the firstregion 6.

FIG. 2 shows further wear of the cutter 1 after additional use and itwill be seen that the wear flat proceeds more quickly in the softerlayer of the first region 6 of PCD material than in the more wearresistant layer of the second region 8 of the body of PCD material. Thewear has therefore progressed into the first region 6 and, as thematerial of the first region 6 wears faster than the material of thesecond region 8, the angle of the cutting face (denoted by the back rakeangle φ) gradually decreases so that the wear of the first region 6 isgreater than that of the second region 8.

This may have the effect of slowing down the progression of thewear-flat in the chamfer region 20.

FIG. 3 shows a further stage where the wear flat has reached the rakeface 14 as it intersects the chamfer 20 and has also reached theinterface 3 of the first region 6 and the substrate 2. Further wear andretardation of the wear flat in the chamfer region 20 delays thespalling which may occur when the wear flat reaches the corner of thechamfer 20 as denoted by the second hashed line 24.

Once the wear reaches the top of the chamfer 20, this could lead tospalling and, once the wear reaches the interface 3 between thesubstrate 2 and the first region 6, the cutter 1 may have reached theend of its useful working life.

FIG. 4 a is a side view of a conventional cutter showing the wear scarfor a predetermined number of passes. It will be seen that the wear onthis cutter is greater than that on the cutter of FIG. 4 b which is inaccordance with a first embodiment for the same predetermined number ofpasses as that applied to the cutter of FIG. 4 a.

FIG. 5 a is a side view of the conventional cutter of FIG. 4 a after afurther number of passes at which spalling has occurred. It will be seenthat there is extensive spalling damage to the cutter whilst the cutterof FIG. 4 b after the same number of passes applied to the conventionalcutter shows only a small amount of wear.

FIG. 6 is a side view of the embodiment of the cutter of FIG. 4 b aftera further number of passes with the onset of spalling behaviour.

FIG. 7 a is a side view of a further conventional cutter after apredetermined number of passes and FIG. 7 b is a side view of a furtherembodiment of a cutter showing the wear scar for the same predeterminednumber of passes as that applied to the cutter of FIG. 7 a. It will beseen that in the embodiment shown in FIG. 7 b, whilst the wear scar islarger than that shown in FIG. 7 a, the wear is all in the woody region.

FIG. 8 a is a side view of the conventional cutter of FIG. 7 a after afurther number of passes at which spalling has occurred. FIG. 8 b is aside view of the embodiment of a cutter shown in FIG. 7 b after the samenumber of passes applied to the conventional cutter of FIG. 8 a. In theembodiment of FIG. 8 b, the cutter has maintained a sharp cutting edgealthough the wear scar is larger than that in the cutter of FIG. 8 a.

FIG. 9 is a side view of the embodiment of the cutter of FIG. 8 b aftera further number of passes. The cutter has failed due to the large wearscar although a sharp cutting edge is still visible.

The material forming the second region 8 is chosen to be significantlymore wear resistant than the material forming the first region 6. Thesignificantly lower wear resistance of the first region 6 assists inenabling a desired wear pattern to be created in use.

The cutter 1 may be fabricated as follows.

As used herein, a “green body” is a body comprising grains to besintered and a means of holding the grains together, such as a binder,for example an organic binder. Embodiments of superhard constructionsmay be made by a method including preparing a green body comprisinggrains of superhard material and a binder, such as an organic binder.The green body may also comprise catalyst material for promoting thesintering of the superhard grains. The green body may be made bycombining the grains with the binder and forming them into a body havingsubstantially the same general shape as that of the intended sinteredbody, and drying the binder. At least some of the binder material may beremoved by, for example, burning it off. The green body may be formed bya method including a compaction process, injection or other molding,extrusion, deposition modelling or other methods. The green body may beformed from components comprising the grains and a binder, thecomponents being in the form of sheets, blocks or discs, for example,and the green body may itself be formed from green bodies. For example,the green body for the superhard construction may be formed fromdistinct green bodies for each of the respective regions 6, 8, which maybe formed separately into generally the intended shapes of therespective regions and combined to form a boundary defined by a contactinterface.

One embodiment of a method for making a green body includes providingtape cast sheets, each sheet comprising a plurality of diamond grainsbonded together by a binder, such as a water-based organic binder, andstacking the sheets on top of one another and on top of a support body.Different sheets comprising diamond grains having different sizedistributions, diamond content or additives may be selectively stackedto achieve a desired structure. The sheets may be made by a method knownin the art, such as extrusion or tape casting methods, wherein slurrycomprising diamond grains and a binder material is laid onto a surfaceand allowed to dry. Other methods for making diamond-bearing sheets mayalso be used, such as described in U.S. Pat. Nos. 5,766,394 and6,446,740. Alternative methods for depositing diamond-bearing layersinclude spraying methods, such as thermal spraying.

A green body for the superhard construction may be placed onto asubstrate, such as a cemented carbide substrate to form a pre-sinterassembly, which may be encapsulated in a capsule for an ultra-highpressure furnace, as is known in the art. The substrate may provide asource of catalyst material for promoting the sintering of the superhardgrains. In some embodiments, the superhard grains may be diamond grainsand the substrate may be cobalt-cemented tungsten carbide, the cobalt inthe substrate being a source of catalyst for sintering the diamondgrains. The pre-sinter assembly may comprise an additional source ofcatalyst material.

In one version, the method may include loading the capsule comprising apre-sinter assembly into a press and subjecting the green body to anultra-high pressure and a temperature at which the superhard material isthermodynamically stable to sinter the superhard grains. In oneembodiment, the green body may comprise diamond grains and the pressureis at least about 5 GPa and the temperature is at least about 1,300degrees centigrade. In one embodiment, the green body may comprise cBNgrains and the pressure is at least about 3 GPa and the temperature isat least about 900 degrees centigrade.

An embodiment of a superhard construction may be made by a methodincluding providing a PCD structure and a diamond composite structure,forming each structure into the respective complementary shapes,assembling the PCD structure and the diamond composite structure onto acemented carbide substrate to form an unjoined assembly, and subjectingthe unjoined assembly to a pressure of at least about 5.5 GPa and atemperature of at least about 1,250 degrees centigrade to form a PCDconstruction.

A version of the method may include making a diamond composite structureby means of a method disclosed, for example, in PCT applicationpublication number WO2009/128034 for making a super-hard enhancedhard-metal material. A powder blend comprising diamond particles,particles of carbide material and a metal binder material, such ascobalt may be prepared by combining these particles and blending themtogether. Any effective powder preparation technology may be used toblend the powders, such as wet or dry multi-directional mixing,planetary ball milling and high shear mixing with a homogenizer. In oneembodiment, the mean size of the diamond particles may be at least about50 microns and they may be combined with other particles simply bystirring the powders together by hand. In one version of the method,precursor materials suitable for subsequent conversion into carbidematerial or binder material may be included in the powder blend, and inone version of the method, metal binder material may be introduced in aform suitable for infiltration into a green body. The powder blend maybe deposited in a die or mold and compacted to form a green body, forexample by uni-axial compaction or other compaction method, such as coldisostatic pressing (CIP). The green body may be subjected to a sinteringprocess known in the art for sintering similar materials without thepresence of diamond, such as may be used to sinter cemented tungstencarbide, to form a sintered article. For example, the green body may besintered by means of hot pressing or spark plasma sintering. The diamondparticles may wholly or partially convert to a non-diamond form ofcarbon, such as graphite, depending on the sintering conditions. Thesintered article may be subjected to a subsequent treatment at apressure and temperature at which diamond is thermally stable to convertsome or all of the non-diamond carbon back into diamond and produce adiamond composite structure. An ultra-high pressure furnace well knownin the art of diamond synthesis and the pressure may be at least about5.5 GPa and the temperature may be at least about 1,250 degreescentigrade.

An embodiment of a superhard construction may be made by a methodincluding providing a PCD structure and a precursor structure for adiamond composite structure, forming each structure into the respectivecomplementary shapes, assembling the PCD structure and the diamondcomposite structure onto a cemented carbide substrate to form anunjoined assembly, and subjecting the unjoined assembly to a pressure ofat least about 5.5 GPa and a temperature of at least about 1,250 degreescentigrade to form a PCD construction. The precursor structure maycomprise carbide particles and diamond or non-diamond carbon material,such as graphite, and a binder material comprising a metal, such ascobalt. The precursor structure may be a green body formed by compactinga powder blend comprising particles of diamond or non-diamond carbon andparticles of carbide material and compacting the powder blend.

The present disclosure may be further illustrated by the followingexamples which are not intended to be limiting.

EXAMPLE 1

In one embodiment, ultra-high pressure and temperature may be used tosinter the superhard construction at approximately 6.8 GPa or higher.The resulting top layer, namely the second region 8 may comprisesintered fine grains of multimodal diamond, with average final grainsize of, for example, approximately 0.1 to 10 μm, 1 to 8 μm, 3 to 6 μmor 3.5 to 4.5 μm. This second region 8 may be, for example, between 400μm and 1000 μm thick, or between 600 μm and 800 μm thick. The verticalheight of the chamfer 20 may be, for example, between 350 μm and 450 μm,such as around 400 μm. The first region 6 may comprise less wearresistant sintered coarser grains of multimodal diamond of average finalsize of, for example, approximately 6.0 to 20 μm, 8 to 17 μm, 6 to 17 μmor 8.0 to 9.0 μm. This first region 6 may be, for example between about1200 μm and 1800 μm thick, such as between about 1400 μm and 1600 μmthick.

Such an embodiment of a PCD compact may, for example, be prepared asfollows. 2.5 g of a first multimodal diamond powder mix having anaverage particle size of approximately 7 μm and 2.5 g of a secondmultimodal diamond powder mix having an average particle size ofapproximately 11 μm and 3 weight percent VC-TiC admix may be preparedand bound into organic tape which is easily removable by pre-heating,using methods well known in the art. Sufficient discs of the first tapeto form a top sintered layer of approximately 600 μm final thickness maybe placed in a Niobium canister, and similarly sufficient discs of thesecond tape to form the underlying sintered layer of approximately 1600μm final thickness may be placed in the canister on top of the firstdiscs. A tungsten carbide substrate is then placed in the Niobiumcanister on top of the second discs, the canister is sealed and thenheat-treated to remove the organic binders. The canister may be treatedat ultra-high pressure and temperature (for example at approximately1600° C. and 6.8 GPa or greater). After sintering, the PCD cutters maybe ground to size including a 45° chamfer of approximately 0.4 mm heighton the body of PCD material so produced. Cutters produced according tothe above have been subjected to wear tests (as shown in FIGS. 4 b, 5 band 6) by suitably preparing them as would be appreciated by the skilledperson, to machine a granite block mounted on a vertical turret millingapparatus and counting the number of passes before failure. The averagenumber of passes achieved was approximately 65% better than that of acommercial benchmark, namely that shown and described above withreference to FIGS. 4 a and 5 a.

EXAMPLE 2

In a further embodiment, the second region 8 may comprise coarsesintered grains of multimodal diamond, with average final size ofapproximately 4.5-5.5 μm. In this embodiment, the source diamond may beadmixed with any combination of, for example TiC, TaC, VC, carbonitridesof Ti, Ta, V, in amounts 1% to 6% by weight. An example of such an admixis 2-4% TiC-VC. This second region 8 may be, for example between 400 μmand 1000 μm thick, such as between 600 μm and 800 μm thick. The chamferangle is approximately 45° with vertical height of the chamfer 20 being,for example between around 350 μm and 450 μm, such as around 400 μm. Thefirst region 6 may comprise less wear resistant sintered coarser grainsof multimodal diamond of average final size of approximately 8.0-9.0 μm.This first region 6 may, for example, be between about 1200 μm and 1800μm thick, such as between about 1400 μm and 1600 μm thick.

Such an embodiment of a PCD compact may, for example, be prepared asfollows. 2.5 g of two multimodal diamond powder mixes having averageparticle sizes of approximately 5 μm and approximately 11 μm may beprepared and bound into organic tape easily removed by pre-heating,using methods well known in the art. Sufficient discs of the first tapeto form a top sintered layer of approximately 600 μm final thickness areplaced in a Niobium canister, and similarly sufficient discs of thesecond tape to form the underlying sintered layer of approximately 1600μm final thickness are placed in the canister on top of the first discs.A tungsten carbide substrate is then placed in the Niobium canister ontop of the second discs, the canister is sealed and then heat-treated toremove the organic binders. The canister may be treated at ultra-highpressure and temperature (such as approximately 1600° C. and 6.8 GPa).After sintering, the PCD cutters may be ground to size including a 45°chamfer of 0.4 mm height on the body of the PCD material. Cuttersproduced in this manner were subjected to wear tests by suitablypreparing them as would be appreciated by the skilled person, to machinea granite block mounted on a vertical turret milling apparatus andcounting the number of passes before failure. The average number ofpasses achieved, as illustrated in FIGS. 7 b, 8 b and 9, outperformed acorresponding conventional cutter (as shown in FIGS. 7 a and 8 a) whichhad not been surface-treated by a factor of about three.

Whilst not wishing to be bound by a particular theory, the above resultsindicate that more wear-resistant finer-grain PCD material on lesswear-resistant coarser-grain PCD material may significantly enhance thedurability of the cutter produced according to some embodimentsdescribed herein. The wear starts in the thinner, more wear-resistantlayer of the second region 8 and progresses to the underlying thicker,less wear-resistant layer of the first region 6 which is bonded to thesubstrate 2. Unlike in typical monolayer configurations known in theart, these configuration may assist in diverting the wear scar downwardsinto the barrel of the PCD body, instead of the typical behaviour, inwhich the wear-scar generates cracks which move to the free surfaces ofthe cutter and result in failure through spalling. This has the effectthat the wear behaviour in cutters according to some embodiments mayremain longer in the smooth to “woody” wear region, before eventuallyspalling. Performance may be further improved when the interface 3between the body of PCD material and the substrate 2 is non-planar (notshown).

1. A polycrystalline superhard construction comprising: a body ofpolycrystalline superhard material; a substrate of hard material bondedto the body of polycrystalline superhard material along an interface;wherein the body of polycrystalline superhard material comprises a firstregion and a second region, the first region abutting the substratealong the interface and the second region being bonded to the firstregion along a further interface, the second region defining a rakeface, a cutting edge, a chamfer and at least a part of a flank face, thecutting edge being defined by an edge of the flank face joined to thechamfer, the chamfer extending between the cutting edge and the rakeface; the first region having a first thickness and the second regionhaving a second thickness; the chamfer having a height in a planeparallel to the plane through which the longitudinal axis of thepolycrystalline superhard construction extends, the height of thechamfer being less than the thickness of the second region; the firstregion comprising a material having coarser grains than the material ofthe second region.
 2. A polycrystalline superhard construction accordingto claim 1 wherein the thickness of the first region is greater than thethickness of the second region.
 3. A polycrystalline superhardconstruction according to claim 1, wherein the thickness of the secondregion is up to around 600 microns.
 4. A polycrystalline superhardconstruction according to claim 1, wherein the thickness of the firstregion is around 1200-1800 microns.
 5. A polycrystalline superhardconstruction according to claim 1, wherein the thickness of the secondregion exceeds the height of the chamfer by around between 100 to 400microns.
 6. A polycrystalline superhard construction according to claim1, wherein the height of the chamfer is between around 100-400 microns.7. A polycrystalline superhard construction according to claim 1,wherein the body of polycrystalline superhard material comprisespolycrystalline diamond material.
 8. A polycrystalline superhardconstruction according to claim 7, wherein the average grain size of thediamond grains forming the second region in the body of polycrystallinediamond material is between around 0.1 to 10 microns.
 9. Apolycrystalline superhard construction according to claim 7, wherein theaverage grain size of the diamond grains forming the second region inthe body of polycrystalline diamond material is between around 1 to 8microns.
 10. A polycrystalline superhard construction according to claim7, wherein the average grain size of the diamond grains forming thesecond region in the body of polycrystalline diamond material is betweenaround 3 to 6 microns.
 11. A polycrystalline superhard constructionaccording to claim 7, wherein the average grain size of the diamondgrains forming the first region in the body of polycrystalline diamondmaterial is between around 6 to 20 microns.
 12. A polycrystallinesuperhard construction according to claim 7, wherein the average grainsize of the diamond grains forming the first region in the body ofpolycrystalline diamond material is between around 8 to 17 microns. 13.A polycrystalline superhard construction according to claim 7, whereinthe average grain size of the diamond grains forming the first region inthe body of polycrystalline diamond material is between around 6 to 17microns.
 14. A polycrystalline superhard construction according to claim1, wherein the chamfer angle is approximately 45°.
 15. A polycrystallinesuperhard construction according to claim 1, wherein the interfacebetween the first region and the substrate is substantially non-planar.16. A polycrystalline superhard construction according to claim 1,wherein the substrate comprises cemented carbide.
 17. A cutter forboring into the earth comprising the polycrystalline superhardconstruction according to claim
 1. 18. A PCD element for a rotary shearbit for boring into the earth, for a percussion drill bit or for a pickfor mining or asphalt degradation, comprising the polycrystallinesuperhard construction of claim
 1. 19. A drill bit or a component of adrill bit for boring into the earth, comprising a polycrystallinesuperhard construction according to claim
 1. 20. A method for making apolycrystalline superhard construction as claimed in claim 1, the methodincluding: providing a first plurality of aggregate masses comprisingdiamond grains having a first mean size, at least one second aggregatemass comprising diamond grains having a second mean size; arranging thefirst aggregate mass on the second aggregate mass to form a pre-sinterassembly together with a body of material for forming a substrate; thefirst region comprising a material having coarser grains than thematerial of the second region; and treating the pre-sinter assembly inthe presence of a catalyst material for diamond at an ultra-highpressure and high temperature at which diamond is more thermodynamicallystable than graphite to sinter together the diamond grains and asubstrate bonded thereto along an interface to form an integral PCDconstruction comprising a first region of PCD bonded to a second regionof PCD, the first region being bonded to the substrate; the first regionhaving a first thickness and the second region having a secondthickness; the second region defining a rake face, a cutting edge, andat least a part of a flank face; the method further comprising: forminga chamfer in the flank face, the cutting edge being defined by an edgeof the flank face joined to the chamfer, the chamfer extending betweenthe cutting edge and the rake face, the chamfer having a height in aplane parallel to the plane through which the longitudinal axis of thesuperhard construction extends, the height of the chamfer being lessthan the thickness of the second region.